This invention relates to semiconductor devices and, more specifically, relates to a novel process for fabricating a trench structure for a device in which an array of planar cells are formed in a single silicon wafer and are dielectrically isolated from one another.
It is often desirable to fabricate a semiconductor device formed of large number of cells. Photovoltaic generators (PVG), for example, are well known and are commonly used for producing a control signal for a solid state relay. Such devices employ an LED which is energized by input terminals to irradiate the photosensitive surface of a spaced and insulated photovoltaic device. The output of the photovoltaic device may serve as the input to a switching device, such as a MOS-gated device, typically a power MOSFET or IGBT, which has load terminals which are switched "on" in response to the energization of the LED. The input and output terminals of the relay are isolated by the gap between the LED and the photovoltaic device. Commonly, the photovoltaic device consists of a large number of series-connected photovoltaic cells in order to produce a voltage sufficiently high to turn on the power switching device. Such devices are well known and are sold under the name "PVI" (photovoltaic isolator) by the International Rectifier Corporation of El Segundo, Calif., the assignee of the present invention.
The plural cell photogenerator can be made in many different ways. One known generator employs a stack or pile of photovoltaic cells as shown in U.S. Pat. Nos. 4,755,697 and 4,996,577, both to Daniel M. Kinzer. Other devices employ a planar array of cells which are junction isolated from one another and are connected in series at their surfaces. Still other devices are known in which individual cells disposed over the surface of a silicon chip are junction-isolated from one another or may be dielectrically isolated, as shown in U.S. Pat. Nos. 4,227,098 and 4,390,790. The prior art devices, however, have the drawback of being expensive to manufacture as well as having low manufacturing yields.
Alternatively, a planar array of photovoltaic generating cells are formed in a dielectrically bonded silicon wafer. A relatively thick "handle" wafer is oxide bonded to, as well as insulated from, a thin device wafer in which the junctions are formed, as shown in U.S. Pat. No. 5,549,762 to the present applicant. This device, however, requires a relatively expensive starting wafer.
It is therefore desirable to produce a photovoltaic generator that can be formed of a large number of insulated cells which can be connected in series to produce a turn-on signal for a power MOS-gated device but which is easily manufactured and integrated with the MOS-gated device using existing reliable processing equipment and techniques. More specifically, it is desirable to produce a photovoltaic generator comprised of planar photovoltaic generating cells formed in a single wafer in which the cells are dielectrically isolated by a trench structure in which the trenches are of a predefined depth and are filled with an insulating material to dielectrically insulate each of the cells. The fabrication process for such a device is described, for example, in U.S. application Serial No. (IR-1457), which is incorporated herein by reference.
During the fabrication of such a trench structure, however, the oxide or other dielectric material that is grown or deposited in the trench often is thicker at the upper portion of the trench than in the lower regions of the trench. As a result, the deposited or grown insulating material may pinch-off and close the upper opening of the trench while leaving a lower region in the trench unfilled. The gaps in the trench weaken the insulating properties of the trench and can produce devices with lower voltage ratings as well as poor mechanical properties. This problem is further exacerbated when the trench is etched with the upper part of the walls at a re-entrant angle which produces a "pinch-off" region in which the upper opening of the trench is closed off while leaving a lower region in the trench unfilled.
It is therefore desirable that the fabrication process for the device produce a trench structure in which the trench is completely filled.